ReseaRch highlights
Raoult and co-workers previously virus that was 50 nm in size, which derived challenged the field of virology by they named Sputnik. from mimivirus identifying a virus that was so large Sputnik did not grow when or mamavirus, which its size questioned the definition of inoculated into Acanthamoeba cas- indicates that this virophage par- a virus. Now, publishing in Nature, tellanii. However, it did grow in ticipates in gene-transfer processes La Scola, Desnues, Raoult and col- A. castellanii cells that were co- and mediates lateral gene transfer leagues describe a new strain of the infected with mamavirus, and its between giant viruses. giant mimivirus that has its own effect on mamavirus was deleteri- The other five genes encode viral infection. ous. The growth of Sputnik in the proteins that are homologues of an The previously identified giant viral factories of mamavirus archaeal virus integrase, a predicted Acanthamoeba polyphaga mimivirus, significantly impaired the produc- primase–helicase, a packaging which only grows in amoebae, tion of normal mamavirus virions, ATPase, a distant homologue of bac- was isolated from a cooling tower which indicates that Sputnik is a terial insertion sequence transposase in Bradford, United Kingdom. genuine parasite of mamavirus. DNA-binding subunit and a zinc- Mimivirus was visible under an Although satellite viruses of animals ribbon protein. Interestingly, the optical microscope and became the or plants also need a helper virus to closest homologues of the last four largest known virus. Now, following replicate, this is the first time that a proteins were detected in the Global inoculation of A. polyphaga with virus has been identified that uses Ocean Sampling Expedition, which The growth water from a cooling tower in Paris, the viral factory of another virus indicates that Sputnik is the first of Sputnik in France, La Scola et al. have identified to propagate at the expense of the member of a new family of viruses the giant viral a new strain of mimivirus that is even helper virus. The authors have pro- that is abundantly represented in factories of larger. They named this new strain posed that Sputnik is a ‘virophage’ marine metagenomic sequences. mamavirus mamavirus and its main features owing to its functional analogy with Rachel Smallridge significantly closely resemble those of mimivirus, bacteriophages. including the formation of giant viral The Sputnik genome is an 18,343 origiNAl rESEArCH PAPEr La Scola, B. et al. impaired the factories in host cells. Fascinatingly, bp circular double-stranded DNA The virophage as a unique parasite of the giant production of mimivirus. Nature 6 Aug 2008 (doi:10.1038/ however, they found that mamavi- with 21 predicted protein-encoding nature07218) normal mamavirus rus was not alone in the infected genes. Of these, eight encode FurtHEr rEAdiNg Raoult, D. & Forterre, P. A. polyphaga cells — also present proteins with homologues. Three Redefining viruses: lessons from Mimivirus. virions... Nature Rev. Microbiol. 6, 315–319 (2008) was an unknown, small icosahedral of these eight genes seem to be
NATURE REvIEwS | microbiology vOLUME 6 | MONTH 2008